In frame transmissions between devices in a wireless communication system that implements power saving, a power saving sequence begins with the transmission of an unaggregated power save frame, and terminates when the last scheduled uplink (UL) transmission ends.
An example of such a wireless communication system is IEEE 802.11n. The IEEE 802.11 Task Group n (TGn) provides a high data rate wireless local area network (WLAN) standard which allows a maximum throughput of at least 130 Mbps. A typical IEEE 802.11n WLAN includes an access point (AP) and at least one station (STA). An amended TGn draft specification (IEEE P802.11n/D1.0 (March 2006), “Amendment: Wireless LAN MAC and PHY specifications: Enhancement for Higher Throughputs”), incorporated herein by reference, provides a power management scheme termed PSMP. In frame transmissions between an AP and a STA in an IEEE 802.11n WLAN that implements PSMP, a PSMP sequence begins with the transmission of an unaggregated PSMP frame, and terminates when the last scheduled uplink (UL) transmission ends.
As shown in FIG. 1, such a PSMP frame 10 comprises a PSMP control header 12 and one or more STA information (STA info) fields 14. Each STA info field 14 carries timing details 16 of scheduled UL periods and timing details 18 of scheduled downlink (DL) periods. Each STA info field 14 also includes sub-fields 20 and 22 for UL and DL start time offsets, respectively, which help in determining the exact time when the UL and DL periods start. Each STA info field 14 further includes the STA identification (STA ID) field 24 that identifies a corresponding STA that the STA info field 14 is intended for. Based on the information in each STA info field 14, the STA that is identified in the STA ID field 24 can determine exactly when to sleep and when to wake up for data communications (i.e., data transmission/reception).
FIG. 2 shows a typical PSMP communication sequence 25 for a WLAN with an AP and multiple stations (e.g., STA1, STA2 and STA3). The PSMP frame 10 includes STA info fields 14 for the stations STA1, STA2 and STA3. The STA info fields 14 indicate the scheduled durations for the DL periods 26 (for communicating the DL data packets 21 to the STAs), and the UL periods 27 (for communicating the UL data packets 23 to the AP), for each of the stations, STA1, STA2 and STA3. The DL periods 26 are separated by intervals 28 (e.g., short interframe space (SIFS) intervals and/or reduced interframe space (RIFS) intervals). Similarly, the UL periods 27 are separated by the intervals 28.
In one example, multiple streams such as MP3 audio, email, chat, VOIP, etc., are aggregated for a single STA in a PSMP sequence. The PSMP control header 12 further includes a specialized PSMP field which: (1) when set to 1, indicates whether the PSMP sequence is followed immediately by another PSMP sequence, and (2) when set to 0, indicates that the current PSMP sequence is the last in the current service period.
The Medium Access Control Protocol Data Unit (MPDU) for a PSMP frame is a broadcast/multicast packet. As a result, it is highly likely that such a packet can collide with other simultaneous transmissions. Conventional approaches suitable for protecting unicast packets are not effective in protecting the PSMP frame against collisions. For example, an RTS/CTS exchange has conventionally been used to protect unicast packet delivery. However, this is not suitable for broadcast/multicast packet such as the PSMP frame.
Another conventional approach is to transmit the PSMP MPDU at a base rate. Yet another conventional approach is to precede the transmission of the PSMP MPDU by a CTS (i.e., CTS-to-self) frame with the receiver address (RA) field set to the initiator's (e.g., the AP's) own media access control (MAC) address. However, neither approach can fully protect the PSMP frame. Therefore, it is quite possible that the PSMP frame is not correctly received by one or more PSMP-enabled STAs in a PSMP sequence. As such, there is a need for a reliable PSMP frame transmission approach in broadcast/multicast wireless applications.